The demand for high capacity rechargeable electrochemical cells is strong. Many areas of application, such as aerospace, medical devices, portable electronics, and automotive, require high gravimetric and/or volumetric capacity cells. Lithium ion technology represents a significant improvement in this regard. However, to date, application of this technology has been primarily limited to graphite negative electrodes, and graphite has a theoretical capacity of only 372 mAh/g during lithiation.
Silicon, germanium, tin, and many other materials are desirable negative active electrode materials because of their high electrochemical capacities. For example, the theoretical capacity of silicon during lithiation is estimated to be about 4,200 mAh/g. However, many of these materials have not been widely adopted because of their poor cycle life performance. One reason for this poor performance is substantial volumetric change during cycling. Silicon, for example, swells by as much as 400% when it is lithiated to its maximum capacity. Volume changes of such magnitude can cause considerable stress in high capacity active material structures and their solid electrolyte interphase (SEI) layers. This stress, in turn, results in fractures and pulverization and significant capacity fading. It is believed that SEI layer formation is accompanied by considerable loss of lithium ions. Further, it is believed that an SEI layer formed on high capacity active material structures continues to break and reform after initial formation, as the structures are repeatedly lithiated and delithiated. This continuous SEI layer formation also continues to consume lithium ions and other electrolyte components. Furthermore, portions of the SEI layer may become unusually thick as newly-formed SEI materials move around in this dynamic SEI layer. These phenomena are believed to cause at least some capacity fading. Overall, capacity fading is generally much higher for cells built with high capacity materials than cells built with conventional graphite, which is attributable, at least in part, to unstable SEI layers formed over high capacity active material structures.